There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biological information. In particular, when conducted in microfluidic volumes, complicated biochemical reactions may be carried out using very small volumes of liquid. Among other benefits, microfluidic systems improve the response time of reactions, minimize sample volume, and lower reagent consumption. When volatile or hazardous materials are used or generated, performing reactions in microfluidic volumes also enhances safety and reduces disposal quantities.
One useful analytical process that may be performed using microfluidic devices is chromatography—a process routinely performed in various industrial and academic settings. Chromatography encompasses a number of methods that are used for separating closely related components of mixtures. In fact, chromatography has many applications including separation, identification, purification, and quantification of compounds within various mixtures. Chromatography is a physical method of separation wherein components partition between two phases: a stationary phase and a mobile phase. Samples are carried by a mobile phase through a bed of stationary phase and separated into their constituent components. Typically, the effluent from the separation column will be subjected to one or more analytical processes, including, but not limited to, optical detection, such as ultraviolet/visible light absorbance.
In order for optical analysis to yield useful results, an optical path transparent to the frequency of the illumination source must be provided. Moreover, the optical path must pass through a sufficient depth of the fluid being analyzed to provide a clear signal. Typical microfluidic devices may limit the ability to provide either of these desirable qualities for a number of reasons. For instance, many of the materials used to fabricate microfluidic devices, such as silicon, PEEK, and polyimide, may be opaque to or otherwise interfere with the transmission of desirable illumination frequencies, such as visible or ultraviolet light. Also, the dimensions of typical microfluidic conduits are such that any light path passing the depth or width of the conduit passes through a very small amount of fluid, thereby limiting the amount of information that may be acquired therefrom.
One proposed solution is to provide a microfluidic conduit with a path that provides an optical detection path along a length (as a opposed to a width or depth) of the fluid conduit, as disclosed in U.S. Pat. No. 5,757,482 to Fuchs et al. (“Fuchs”) and U.S. Pat. No. 4,823,168 to Kamahori et al. (“Kamahori”). Fuchs discloses a micro-machined conduit extending vertically through a substrate that is the sandwiched between optically transmissive layers, providing a usefully long optical path length for optical detection of analytes. Kamahori discloses a cell body assemble from two etched halves that form a “Z”-shaped channel, bounded on either end with optically transmissive windows to provide an optical path through a length of the fluid channel. Both the Fuchs and Kamahori devices are limited, however, in that multiple components must be assembled to fabricate the device. In addition to adding complex and time-consuming assembly procedures (for example, to ensure precise alignment of the components), the bonding together of such components may require the use of adhesives that might contaminate the analyte stream. To the extent that adhesiveless bonding methods may be used, such methods are typically complex, often requiring substantial surface treatment of one or more of the components to facilitate bonding of dissimilar materials.
Thus, it would be desirable to provide a device for performing optical detection of analytes in a fluid stream that minimizes the number of components used to fabricate the device, is simple to manufacture, and provides the desired optical transmission through an optical path length that is substantially longer than the depth or width of a microfluidic channel.